MEMS rotation-rate sensors (microelectromechanical systems) require a high mechanical quality of the mechanical MEMS element. Said quality is essentially defined by an internal pressure in the MEMS cavity. Because of friction between a residual gas and a mechanical oscillator structure, energy is continuously drawn from the system, which limits the maximum quality in the mechanical system.
In order to set a low internal pressure (i.e., a high quality of the sensor), sensor wafer and cap wafer are bonded in a chamber under a desired internal pressure. This method offers the possibility of setting internal pressures greater than approx. 1 mbar. Residual gas and degassing of the surface limit the achievable minimum pressure.
In a conventional method, it is possible to reduce the internal pressure with the aid of a pasty getter material that is applied in a planar manner in the cap of the rotation-rate sensor, and which is chemically activated. This method is used in combination MEMS systems (having two separate cavities, for example rotation-rate sensor and acceleration sensor) in order to provide a low pressure for the rotation-rate sensor and at the same time a higher pressure for the acceleration sensor. The sensor wafer and the cap wafer are bonded at the target pressure of the acceleration sensor. In this manner, the required internal pressure is set for the acceleration sensor. The getter material in the cap wafer then binds the residual gas in the rotation-rate sensor cavity. It is thus possible to reduce the required internal pressure in the rotation-rate sensor to approx. 1 mbar.
Conventional macroscopically developed vacuum systems are furthermore available, in which getter pumps are used with metallic getter materials (for example titanium), a residual gas being gettered away by evaporating the getter material and a vacuum being thereby produced.